or you can use northeastern and you need Radio-shack PCB Etchant solution cost about $9.95 .Ok so you have all the item let get rolling .First get two plastic can one for water one for the solution.Now cut the metal to size you needed to do roof you can cut then to scale after your done . Now wear rubber glove and in In a well-ventilated area with a fan or open air vented room or out side.Always wear goggles, gloves, and do not inhale the fumes. Do not use any metal containers, measuring cups .Put the metal in the solution and wait 30sec to a min your see it start to boil and smoke at this time work fast and remove it with Dermal Anchor Piercing Hemostat Forcep TOOL .or needle nose and into the water for 5min then put on paper towel to dry it be dark gray first but it turn into rust when drying . now clean up can and wash hand good .Please read This etching solution, while made with common chemicals, should command respect. It is dangerous to yourself and surroundings, treat it with respect.Hi all long time .OK i added a you tube video on this .it at http://www.youtube.com/watch?v=dQTbcXwkro4

Ferric Chloride is the chemical we are talking about and it is rather nasty. It should be used outdoors, avoid splashing it ony you , avoid the fumes and DON'T pour it down the sink drain when done. Here is one cautionary statement on the net about it.
Iron(III) chloride is toxic, highly corrosive and acidic. The anhydrous material is a powerful dehydrating agent.

There is a way to get the same effect without all the fuss and hazard.
The aluminum panel intended to be "rusted' can be coated with olive oil, held with tweezers or pliers a few inches above the flame on a gas stove -- when the olive oil begins to smoke it will turn brown and rusty looking.

While we are talking about corrugated metal let me show off a tool you can make at home to make all the corrugated metal you want from heavy duty aluninum foil --- Reynolds works the best-- its just one of those things where one does not shop for price --kinda like Mac and Cheese -- just get Kraft and be done with it!
Here is the tool .

It is easy to make , it is 1 inch wide X1&1/4 inch tall, all you need is 1/4 inch sq. brass , 1/8 x1/4 brass, for HO a piece of 2/56 threaded rod( RC airplane shop will have for sure) a couple of 2/56 X 3/4 screws , I used allens because they make good thumb screws . and a very small amount of K& S brass channel , for the side guides and for the "pressure pads" . Looking at the pics you shold figure out how it all goes together. One thing you can't see is that the free floating 2/56 rod has flat spots ground on each end to keep it aligned and not turning in the channels so that the two threads will mesh petrfectly to make the corrugations. For different scales just vary the size of the tool and the thread per inch figure .
Have fun! Dave